The subject invention relates to the design of a broadband imaging spectrometer for use in thin film measurement and general spectroscopic applications. The invention is broadly applicable to the field of optical metrology, particularly optical metrology tools for performing measurements of patterned thin films on semiconductor integrated circuits.
The use of thin film measurement technologies such as spectroscopic ellipsometry [SE], broadband reflectometry [BBR] and visible light reflectometry [VR] is well established. These technologies typically use a spectrometer to simultaneously gather information about the sample under test at different wavelengths. Examples in the prior art include U.S. Pat. No. 6,278,519 and U.S. Pat. No. 5,910,842 incorporated herein by reference. For optical wafer metrology the wavelength region of interest spans the vacuum ultra-violet [VUV] and near infrared [NIR].
Ideally the spectrometer has the following characteristics:
a) High efficiency over the desired wavelength range. This implies large dynamic range and high wafer throughput. This may permit low power light sources to be used reducing thermal loading of the optical system permitting a simplified design for the thermal management system and the optical mounts. All of these effects combine to improve metrology system performance at reduced cost of ownership.
b) Low spatial distortion. This implies that the detected light comes to sharp focus and forms a small spot size on the detector. Low spatial distortion implies good chromatic separation. This in turn helps minimize xe2x80x9ccross-talkxe2x80x9d between detected wavelengths and improves the accuracy and resolution of the spectrometer.
c) Low chromatic distortion. Low chromatic distortion implies the spot size is consistently small over the desired wavelength range typically from VUV to IR. This minimizes the potential for xe2x80x9ccross-talkxe2x80x9d between detected wavelengths and improves the accuracy and resolution of the spectrometer.
d) Low scatter. Scatter modifies the spatial dependence of the optical intensity striking the detector. Light is removed from one spectrally separated channel (channel A) and is, potentially, deposited into an adjacent channel (channel B). This artificially reduces the channel A signal and artificially increases the channel B signal. This acts to wash-out the chromatic separation and produces measurement error.
e) The spectrometer design should employ a small number of individual components that are maintained in a robust arrangement that is easy to align optically. This insures high performance, simplifies fabrication, minimizes required maintenance and reduces capital costs.
It is a challenge to design a spectrometer that meets all of these requirements. There are three notable prior art spectrometer designs that do not meet all of the above listed requirements but satisfactorily address at least a subset of the requirements.
The simplest of the prior art designs forms the spectrometer with a single element. The most common implementation employs holographic techniques to form the grating on a concave, usually spherical, surface. In these systems the grating has two functions since it focuses and diffracts the incident light. Since the design has a single element it is relatively easy to align. But, the design suffers from high spatial and chromatic distortions over the wavelength range of VUV to IR.
FIG. 1 illustrates another prior art design, the Fastie-Ebert spectrometer 40 that uses two elements, one large spherical mirror 30 and one plane diffraction grating 32 to focus and disperse the light. Different portions of the mirror 30 are used to (1) reflect and collimate light entering the spectrometer onto the plane grating and (2) focus the dispersed light, diffracted from the grating, into chromatically separated images of the entrance slit in the spectrometer exit plane. It is an inexpensive and commonly used design, but exhibits limited ability to maintain off-axis image quality due to system aberrations including spherical aberration, coma, astigmatism, and a curved focal field.
FIG. 2 illustrates another prior-art design, the Czemy-Turner (CZ) spectrometer 50, that is an improvement over the Fastie-Ebert design. The CZ spectrometer employs three elements, two concave mirrors, 33 and 35, and a single plane diffraction grating 32. The two mirrors function in the same separate capacities as the single spherical mirror of the Fastie-Ebert configuration, i.e., mirror 33 collimates and reflects light entering the spectrometer onto the diffraction grating 32, and mirror 35 focuses the dispersed light diffracted from the grating into chromatically separated images of the entrance slit in the spectrometer exit plane, but the geometry of the mirrors in the Czerny-Turner configuration is flexible. By using an asymmetrical geometry, the Czemy-Turner configuration may be designed to produce a flattened spectral field and good coma correction at a single wavelength. However, spherical aberration and astigmatism will remain at all wavelengths. The design has the further advantage that it can accommodate very large optics.
Each of the three elements in the Czemy-Turner spectrometer must be aligned precisely. In designs that utilize off-axis aspheric mirrors, alignment can be a daunting task. Furthermore, conventional optical fabrication methods cannot be used to fabricate off-axis aspherics. More complex and less robust optical fabrication techniques must be employed which can both increase fabrication costs and reduce performance. For example, diamond turning as the preferred method for manufacturing off-axis aspherics. Diamond turned optics exhibit higher optical scatter than conventionally figured surfaces.
Accordingly it would be desirable to provide a compact spectrometer employing the minimum number of elements exhibiting low optical scatter, reduced spatial distortion and reduced chromatic distortion over the wavelength range spanning the VUV-NIR.
A design for a compact imaging spectrometer for use in thin film metrology and general spectroscopic applications is described. In comparison to prior art designs the spectrometer has reduced spherical aberration, coma and astigmatism. The spectrometer includes an entrance aperture arranged to receive light, a wavelength dispersive element, a single, axially rotationally symmetric, aspheric mirror, a detector and a processor. The use of an aspheric mirror permits the correction of spherical aberration. Coma is corrected by system symmetry. The plane of the detector may be tilted to substantially compensate for residual astigmatism.
Diamond turning is a popular method for fabricating off-axis aspheric surfaces. However, unwanted surface features generated in the diamond turning process limit the optical performance of diamond turned optics. Axially rotationally symmetric aspheres, however, can be fabricated using conventional optical polishing techniques. These techniques produce low scatter surfaces at lower cost than diamond turning. Consequently, the spectrometer of the present invention offers enhanced optical performance at reduced cost.